Liquid propellants can be used in spacecraft, aircraft and other projectiles. Although liquid propellants provide highly efficient thrust, the movement of the propellant inside a fuel tank is often unpredictable. During the process of stage separation, one fuel tank may be separated from the payload and the other fuel tanks of a spacecraft. The vibration caused by the stage separation may be transferred into the remaining fuel tank(s), impacting forces and moments on the fuel. These forces can cause the fuel to move around within the tank in an uncontrolled oscillatory fashion, which may lead to a phenomenon known by the term fuel slosh. Fuel slosh can also be caused by other maneuvers, such as when a spacecraft attempts a change in attitude. Generally, fuel moves as a bulk mass inside the tank, where the upper portion of the fluid produces larger displacement than the lower portion.
Sloshing of fuel or other liquid imparts unwanted forces and moments on the tank's walls, and produces oscillatory forces that can cause a spacecraft to wobble (known as nutation). Nutation tends to increase exponentially with time, and is governed by a Nutation Time Constant. If left unattended, nutation can cause undesirable trajectory change. Fuel slosh can also negatively impact the fuel consumption characteristics of the spacecraft.
Anti-sloshing is the process of damping slosh waves from their resonant frequency condition. Some research has been carried out seeking to minimize the energy dissipation of fuel slosh, and seeking to limit the impact of the slosh momentum on fuel tanks. Scientists have developed several systems to counteract the forces and moments produced by fuel slosh, known collectively as propellant management devices (PMD). These devices include active and passive baffle structures, as well as elastomeric membranes. In some instances, passive baffle structures are placed along the inner walls of the tanks, which apart from minimizing the propellant slosh, also act as formers providing structural integrity to the tank design. In other cases, active baffles are structures that are present within or on top of the liquid surface within the tank. The active baffles move along with the liquid and constrain the slosh behavior of the liquid by the application of external stimuli. Example active baffles are described in commonly owned application PCT/US15/10602 entitled “Floating Active Baffles, System and Method of Slosh Damping Comprising the Same”, which is incorporated herein by reference. Although baffles (both active and passive) provide considerable damping effect on the slosh, baffles add to the overall structural mass of the tank, making the tank heavier and reducing the volume of the tank available for propellant. Another type of PMD includes the use of elastomeric membranes. Elastomeric membranes are often thin polymer structures that are resistant to the effects of hydrocarbons present in the fuel. Passive elastomeric membranes are used as fixed diaphragms inside the tank to provide slosh suppression. The effectiveness of slosh damping may depend on the elastic coefficient of the elastomeric membrane. Active elastomeric membranes may be activated to vibrate using external stimuli.
There remains a need for additional propellant management devices that combine the strengths of the existing devices.